Generally, a motor needs an inverter driver to control the rotor frequency of the motor and regulate the rotation speed of the motor, so as to drive the motor to operate. In order to increase the safety of the motor during the operation period, a parallel inverter driver system has been proposed. The parallel inverter driver system includes a plurality of inverter drivers with their outputs connected in parallel with each other, thereby driving the motor. The parallel inverter driver system is different from the conventional inverter driver system in that the parallel inverter driver system is made by substituting the bulky inverter driver with a number of small-volume inverter drivers and allowing these small-volume inverter drivers to output collaboratively for driving motor. The main features of the parallel inverter driver system are focused on modularity and redundancy. The modularity feature means that the controllers for individual inverter drivers with small capacity are independent from other inverter drivers. Thus, the user can expand or lessen the capacity of the inverter driver system depending on user's demands. The redundancy feature means that in case of malfunction, the damaged inverter driver can be replaced during the operation of the inverter driver system without overhauling the entire system. If the inverter driver system is appropriately designed, the motor can be driven to operate continuously on the condition that the other undamaged inverter drivers are operating normally, while the damaged inverter drivers are replaced without the need of shutting down the entire system.
Referring to FIG. 1, which is an architecture diagram of the traditional parallel inverter driver system of the first type. The traditional parallel inverter driver system of the first category of FIG. 1 includes two parallel-connected inverter drivers INV_1 and INV_2, in which the inverter driver INV_1 is a master inverter driver and the inverter driver INV_2 is a slave inverter driver. Va1*, Vb1*, Vc1* are the voltage commands inputted into the inverter driver INV_1, and Va2*, Vb2*, Vc2* are the voltage commands inputted into the inverter driver INV_2. The master inverter driver INV_1 includes a space-vector modulation (SVM) controller 102, an inductance device 103 and a switch device 104. The space-vector modulation (SVM) controller 102 is configured to convert the voltage commands Va1*, Vb1*, Vc1* into duty cycle signals Ta1*, Tb1*, Tc1*. The switch device 104 is driven by the duty cycle signals Ta1*, Tb1*, Tc1* and a duty cycle signal T01 with a zero-voltage vector to conduct switching operations. Thus, an AC output voltage and an AC output current are generated. The inductance device 103 is connected to the outputs of switch device 104 for suppressing the current oscillation of AC output current and then outputting the AC output current to motor 106. Similarly, the slave inverter driver INV_2 includes a space-vector modulation (SVM) controller 108, an inductance device 109 and a switch device 110. The space-vector modulation (SVM) controller 108 is configured to convert the voltage commands Va2*, Vb2*, Vc2* into duty cycle signals Ta2*, Tb2*, Tc2*, the switch device 110 is driven by the duty cycle signals Ta2*, Tb2*, Tc2* and a control signal, which is derived by adding a duty cycle signal T02 with a zero-voltage vector with an adjustment quantity K, to conduct switching operations. Thus, an AC output voltage and an AC output current are generated. The inductance device 109 is connected to the outputs of switch device 110 for suppressing the current oscillation of AC output current and then outputting the AC output current to motor 106. In this example, the operation of the slave inverter driver INV_2 is adjusted along with the operation of the master inverter driver INV_1. The adjustment quantity K is generated by the operation of an adder 112, a subtractor 114 and a proportional integrator 116.
Referring to FIG. 2, which is an architecture diagram of the traditional parallel inverter driver system of the second type. As shown in FIG. 2, the parallel inverter driver system for driving a motor 106 includes parallel-connected inverter drivers INV_1, . . . , INV_n. Each inverter driver is configured to receive a three-phase voltage command (Va1*, Vb1*, Vc1*), . . . , (Van*, Vbn*, Vcn*). Each inverter driver includes a switch device 200, a sinusoidal pulse-width modulation (SPWM) controller 201, a plurality of operators 202 and 203, an inductance device 205, a plurality of gain controllers P and a plurality of current averagers Ave. The switch device 200 is connected to the outputs of the sinusoidal pulse-width modulation (SPWM) controller 201. The operator 203 is used to perform arithmetical operations to the three-phase voltage command (Va1*, Vb1*, Vc1*), . . . , (Van*, Vbn*, Vcn*) and the compensating voltage command, thereby generating a compensated three-phase voltage command and then outputting the command to the sinusoidal pulse-width modulation (SPWM) controller 201 for converting the compensated three-phase voltage command into duty cycle signals (Ta1*, Tb1*, Tc1*), . . . , (Tan*, Tbn*, Tcn*) to switch the switch device 200. Thus, an AC output voltage and an AC output current are generated. The inductance device 205 in each inverter driver can suppress the current oscillation of AC output current and then outputs the suppressed AC output current to motor 106. The compensating voltage command is generated after the operations of the current averagers Ave, the operators 202, 203 and the gain controllers P.
Referring to FIG. 3, which is an architecture diagram of the traditional parallel inverter driver system of the third type. The only difference between the parallel inverter driver system of FIG. 3 and that of FIG. 2 is that the output phase current of each inverter driver is provided to the gain controller P for calculating a compensating voltage command for the output phase current. The compensating voltage command is fed back to the sinusoidal pulse-width modulation (SPWM) controller 201. The operator 203 is used to perform arithmetical operations to the three-phase voltage command (Va1*, Vb1*, Vc1*), . . . , (Van*, Vbn*, Vcn*) and the compensating voltage command, thereby generating a compensated three-phase voltage command.
The above-mentioned three types of the inverter drivers of the parallel inverter driver systems can use the inductance devices with different reactance according to the power state, thereby suppressing the current oscillation of AC output current. However, when the inverter drivers are operated in high power state, the inductance devices with higher reactance must be employed correspondingly. Due to the inductance devices with higher reactance have larger volumes and heavier weights, the volumes and the costs of the parallel inverter driver systems will be increased correspondingly. Although, the inductance devices can be replaced by the LCL filter in some prior arts, the structure of this conventional system is complicated. In addition, short life electrolytic capacitors must be used in the LCL filter, resulting in the electrolytic capacitors must be changed frequently and the cost of the LCL filter is increased. Consequently, the problem of having higher cost still exists in the parallel inverter driver system. Moreover, when the above-mentioned three types of the inverter drivers of the parallel inverter driver system are operated, the current of each inverter drivers can't be outputted to the motor completely due to the minor deviation existed between any two individual inverter drivers. Consequently, the circulating current is generated through current mutually flow between respective inverter drivers. Although each of the three types parallel inverter driver systems is capable of suppressing the circulating current, the parallel inverter driver systems still exist the problem of the zero-sequence current with reference to the circulating current suppressing, so the suppressing effect is limited.
Therefore, there is a need of providing a circulating current and oscillating current suppressing method and parallel inverter driver system in order to eliminate the above drawbacks.